The present invention relates to an image input apparatus such as a television camera. The present invention also relates to an apparatus using a television camera as an image input apparatus and, more particularly, to an X-ray fluoroscopic and radiographic apparatus such as, for example, a real-time digital radiographic apparatus (hereinunder referred to as a "real-time DR apparatus") for inputting an X-ray image in real time, processing the image, and providing an image for diagnosis.
In the field of an X-ray image diagnosis apparatus for medical use, a DR apparatus for converting an X-ray image into an electric signal, processing the digital data obtained by subjecting the electric signal to A/D conversion, and displaying the processed image for diagnosis has recently been increasingly developed. Particularly, an X-ray television apparatus composed of an X-ray image intensifier (hereinunder referred to as an "X-ray II") used for obtaining a fluoroscopic image (an image utilized for reducing the X-ray dose per image and chiefly for determining the portion to be photographed) and a television camera has long attracted attention because it is capable of readily producing X-ray image information in the form of an electric signal in real time.
An X-ray television camera is now widely used as an input apparatus of a digital fluorographic apparatus (hereinunder referred to as "DF apparatus"), which is one of the real-time DR apparatuses capable of imaging a blood vessel with an excellent contrast resolution by subtraction between the images read before and after the injection of a contrast medium, as disclosed in U.S. Pat. Nos. 4,204,225 and 4,204,226.
The DF apparatus requires not only the above-described fluoroscopic image but also a radiographic image (an X-ray image provided with a good image quality by increasing the X-ray dosage per image to about 1,000 times the dosage used for fluoroscopic monitoring) which is to be observed by a doctor for diagnosis.
In this case, since the radiographic image is more important, the apparatus is required to have high spatial resolution, contrast resolution and, if necessary, time resolution, and a wide range of an X-ray intensity which enables imaging, in other words, a wide dynamic range, and among these, the demand for a high spatial resolution and a wide dynamic range is strong.
As described above, in a real-time DR apparatus represented by a DF apparatus, the importance of a radiographic image is greater than a fluoroscopic image, so that a television camera used for the real-time DR apparatus is also required to have a high resolution, a wide dynamic range, a high signal-to-noise ratio, a high time resolution, etc.
Among these requirements, it will be understood if the number of frames per second (e.g., generally 30 frames/sec in the case of scanning 525 lines) of the television camera used in the real-time DR apparatus is taken into consideration that the demand for the time resolution is easily dealt with.
In the present invention, measures for dealing with the other three demands are provided.
In order to improve the spatial resolution, the following measures are conventionally adopted.
(i) A high-resolution type camera tube 1 inch in diameter (for example, a Saticon (a registered trademark of Nippon Hoso Kyokai), or in some cases, a Plumbicon (a registered trademark of N. V. Philips Gloeilampenfabrieken)) is used.
(ii) The number of scanning lines is increased from 525 to 1,125, as described in Japanese Patent Application Laid-Open No. 61-113432.
In order to enlarge the dynamic range and improve a signal-to-noise ratio, the following measures are conventionally adopted.
(iii) The necessary frequency band is limited as much as possible so as to reduce the amount of noise.
(iv) The stray capacitances from the camera tube to the preamplifier are reduced so as to reduce the amount of noise.
(v) A signal current from the camera tube is increased.
Among these, since the measures (i) and (iv) largely depend on the characteristics of the device and the parts such as a camera tube, an FET (field-effect transistor) and a resistor adopted, and the mounting technique therefor, the selection and the mounting technique of the device or the parts are taken into adequate consideration.
In the measures (ii) and (iii), the number of scanning lines and the frequency band are necessarily determined by the specification of the DR apparatus, namely, by the number of images photographed per second and the number of pixels per image.
However, with respect to the measure (ii), although it is easy to increase the number of scanning lines itself, it is a problem whether or not the camera tube adopted has a spatial frequency characteristic which corresponds to the increase in the scanning lines.
Use of a high-resolution type camera tube is therefore insufficient, and a camera tube having as large a diameter as possible which allows a large scanning area on the surface of a photoconductive layer or the surface of a target is adopted.
In the measure (v), it is necessary to increase the amount of charge stored on the surface of a photoconductive layer by increasing the amount of incident light falling onto the camera tube. For this purpose, it is necessary to increase the static capacitance C.sub.s of a photoconductive layer or the voltage applied to the surface of a photoconductive layer, in other words, a signal electrode voltage, or the target voltage V.sub.T of the camera tube.
In order to increase C.sub.s, the thickness d of a photoconductive layer is reduced, the dielectric constant .epsilon..sub.s is increased and the scanning area A.sub.2 is increased.
Among these, d, .epsilon..sub.s and V.sub.T are determined by the characteristics of the photoconductive layer, and A.sub.s is determined by the diameter of the camera tube. Therefore, a camera tube having as large a diameter as possible is used in the same way as in the case of realizing the measure (ii).
As described above, in order to improve the resolution, the dynamic range and the signal-to-noise ratio of a television camera used for a real-time DR apparatus (hereinunder referred to as "television camera for DR"), the diameter of the camera tube is increased and other measures are adopted in the prior art in addition to the selection of a camera tube and the improvement of the circuit and the mounting technique.
As a large-diameter camera tube, a camera tube having a diameter of 2/3 to 1 inch or 1.5 inches, in some cases, is used.
The increase in the diameter of a camera tube, however, is not always advantageous in that the cost of not only the device but also the television camera using the camera tube and further the DR apparatus as a whole is raised.
Still more, new development of a large-diameter camera tube involves a large risk and it is difficult to determine whether or not the thus-developed camera tube will be appropriate.